4.3 Spasticity Assessment & Multimodal Management

Spasticity Assessment & Multimodal Management

Spasticity is defined as a velocity-dependent increase in tonic stretch reflexes (muscle tone) with exaggerated tendon jerks, resulting from hyperexcitability of the stretch reflex as one component of the upper motor neuron (UMN) syndrome. Effective clinical management requires precise clinical quantification, tailored oral antispasmodics, focal chemodenervation, and advanced intrathecal interventions.

Clinical Spasticity Assessment Scales

Modified Ashworth Scale (MAS)

The Modified Ashworth Scale (MAS) is the most widely utilized clinical tool for assessing passive muscle resistance to stretch. Passive joint movement is performed at a quick velocity:

• Grade 0: No increase in muscle tone.
• Grade 1: Slight increase in muscle tone, manifested by a catch and release or by minimal resistance at the end of the range of motion (ROM) when the affected part(s) is moved in flexion or extension.
• Grade 1+: Slight increase in muscle tone, manifested by a catch, followed by minimal resistance throughout the remainder (less than half) of the ROM.
• Grade 2: More marked increase in muscle tone through most of the ROM, but affected part(s) easily moved.
• Grade 3: Considerable increase in muscle tone, passive movement difficult.
• Grade 4: Affected part(s) rigid in flexion or extension.

Modified Tardieu Scale (MTS)

Unlike the MAS, the Modified Tardieu Scale (MTS) directly accounts for velocity dependency, allowing clinicians to distinguish dynamic neural spasticity from fixed structural tissue contracture.

• Velocities of Stretch:
  • V1: As slow as possible (evaluates passive range of motion without triggering stretch reflex).
  • V2: Speed of the limb segment falling under gravity.
  • V3: As fast as possible (faster than the rate of gravity fall, triggering stretch reflex).
• Key Angle Parameters:
  • R2 Angle: Full passive range of motion achieved at slow velocity (V1).
  • R1 Angle: Angle of muscle catch or clonus elicited at fast velocity (V3).
  • Dynamic Catch Zone (R2 minus R1): A large difference between R2 and R1 (large R2 - R1) indicates a large dynamic neural spastic component that will respond favorably to botulinum toxin or intrathecal baclofen. Conversely, an R1 equal to R2 (R2 - R1 = 0) indicates fixed biomechanical contracture, requiring serial casting, splinting, or surgical tendon release rather than antispasmodic medications.

Oral Antispasmodic Pharmacotherapy

Oral antispasmodics are indicated for widespread or generalized spasticity. Selection is guided by mechanism of action, organ toxicity, and side effect profiles.

Clinical Pharmacology Pearls

• Baclofen: Agonizes presynaptic GABA-B receptors, restricting voltage-gated calcium influx and inhibiting excitatory neurotransmitter release in the spinal cord. Abrupt oral withdrawal causes rebound spasticity, hyperpyrexia, and seizures.
• Tizanidine: Central alpha-2 adrenergic agonist structurally related to clonidine. Reduces release of excitatory amino acids (aspartate/glutamate) from spinal interneurons. Causes significant sedation and orthostatic hypotension. Hepatotoxicity risk requires monitoring baseline and periodic AST/ALT levels.
• Dantrolene Sodium: The only direct-acting peripheral muscle relaxant. Binds the ryanodine receptor (RYR1) on the sarcoplasmic reticulum of skeletal muscle cells, blocking intracellular calcium release required for muscle contraction. It spares smooth and cardiac muscle. Because it works peripherally, it produces less central sedation, but carries a Black Box Warning for severe, potentially fatal hepatotoxicity.
• Diazepam: Enhances GABA-A receptor postsynaptic chloride conductance. Highly sedating; impairs motor learning and cognitive recovery after stroke and TBI.

Botulinum Toxin Chemodenervation

Botulinum neurotoxin (BoNT) is the gold standard intervention for focal spasticity (e.g., spastic equinovarus foot, flexed elbow, clenched fist).

Mechanism of Action

BoNT blocks presynaptic release of acetylcholine (ACh) at the neuromuscular junction. The toxin heavy chain binds motor nerve terminal acceptors, endocytoses the toxin, and the light chain cleaves specific SNARE docking proteins:

• Serotype A (OnabotulinumtoxinA, AbobotulinumtoxinA, IncobotulinumtoxinA): Cleaves SNAP-25 (Synaptosomal-associated protein 25).
• Serotype B (RimabotulinumtoxinB): Cleaves VAMP / Synaptobrevin (Vesicle-associated membrane protein).

Dosing and Clinical Timeline

• Clinical Timeline: Therapeutic onset occurs within 3 to 7 days, reaching peak effect at 2 to 4 weeks, and lasting 3 to 4 months (12 to 16 weeks) as functional nerve sprouting restores ACh release.
• Dosing Parameters: For OnabotulinumtoxinA (Botox), the maximum recommended total body dose per injection session in adults is 400 Units (or 10–12 U/kg in pediatric patients). Higher doses increase the risk of systemic toxin spread, presenting with generalized muscle weakness, ptosis, dysphagia, or respiratory distress.

Intrathecal Baclofen (ITB) Therapy

Intrathecal Baclofen (ITB) therapy delivers baclofen directly into the subarachnoid CSF space via a surgically implanted subcutaneous abdominal pump and intrathecal catheter. ITB achieves CSF drug concentrations 10-fold higher than oral administration while utilizing less than 1% of the oral daily dose, bypassing systemic side effects.

• Indications: Severe, intractable lower extremity spasticity (e.g., spinal cord injury, multiple sclerosis, cerebral palsy, severe stroke) unresponsive to max-tolerated oral agents.
• Catheter Tip Placement: Lumbar puncture with tip positioned at T6–T10 for lower extremity spasticity, or C1–C4 for upper extremity and severe quadriparetic spasticity.

ITB Withdrawal vs. ITB Overdose Emergency

Recognizing and managing ITB emergencies is critical for board examinations and clinical practice:

• ITB Withdrawal (Life-Threatening Emergency):
  • Causes: Catheter dislodgement, kink, fracture, pump mechanical failure, low reservoir, or battery depletion.
  • Clinical Presentation: Rapid loss of GABAergic inhibition causes severe rebound spasticity, muscle rigidity, hyperpyrexia (fever >40°C), tachycardia, labile blood pressure, altered mental status, seizures, rhabdomyolysis, acute kidney failure, disseminated intravascular coagulation (DIC), and death (mimicking neuroleptic malignant syndrome or autonomic dysreflexia).
  • Emergency Management: Restore GABAergic tone immediately. Administer high-dose IV benzodiazepines (diazepam, midazolam), high-dose oral/enteral baclofen, supportive fluid resuscitation, and arrange emergent neurosurgical/physiatric pump interrogation and catheter exploration.
• ITB Overdose:
  • Causes: Pump programming errors or unintended bolus.
  • Clinical Presentation: Severe hypotonia ("floppy patient"), somnolence, coma, hypothermia, respiratory depression, bradycardia, and seizures.
  • Management: Mechanical ventilation/airway protection, intravenous physostigmine (1–2 mg IV), and emergency lumbar puncture CSF drainage (removing 30–40 mL of CSF to clear drug).